Transdermal drug delivery systems are used to deliver an active pharmaceutical ingredient (API) or drug to a patient over an extended time period, typically from a period of hours up to about 7 days. Over the years, various types of transdermal drug delivery systems or patches have entered the market, including reservoir-type systems and monolithic matrix-type systems. Reservoir type systems are those that contain a drug reservoir embedded between an impervious backing layer and a rate controlling membrane or additional layers of adhesives, where the drug is in the form of a solution, a suspension, a gel, or drug-in-adhesive (DIA) matrix. Reservoir systems use a skin-contacting adhesive which is applied to the skin-facing surface of the system so the patch can be adhered to a patient's skin. Monolithic DIA systems, on the other hand, are systems in which the drug is contained within an adhesive matrix that functions as both the adhesive layer for securing the patch to the skin and the drug-containing layer. The DIA layer is protected with an impervious backing or film on a surface of the matrix opposite the skin-contacting surface. However, a problem with monolithic DIA transdermal systems is that the diffusion of the drug through the skin of the patient and into the bloodstream is often initially high and exhibits a rapid decrease over time so that the drug delivery is not controlled at the desired rate. Such a phenomenon is known as depletion and can lead to initial over-dosing of a drug or under-dosing of a drug at the end of the patch's application period, where neither is beneficial to the patient.
In consideration of the aforementioned problem, a need exists for a transdermal drug delivery system where the rate of delivery of the drug is controlled so that the delivery is generally constant over an extended period of time.